I just bought an BMWAUC E92 335i M Sport 57REG for £14k OTR. 54k and has Nav PDC etc.
Considering that price includes a 1 year AUC warranty, 1 year tax, new rear brake discs/pads, front pads, 1 new tyre + 3 vgc and some other stuff I forgot what would you imagine the private sale value of this car is? Way less than what Autotrader might make you think.

I have been watching Autotrader on a daily basis for around a year now and it is a poor indication of the price you will actually have to pay as a buyer at this time of year. I made offers to plenty of dealers in the past few months knowing full well they'd be knocking a grand off the car in a month or so anyway. 2/3 of those cars are still on Autotrader and below the prices I offered I have seen BMW dealers cut £3.5k from a price over night, and mine had dropped £2k from the original listing price.

I don't know what advice to give you as a private seller. It is a buyers market at the moment. That may well change from March onward, but for sure around Christmas time most people are short of cash as it is. Judging by what Webuyanycar offered you though you should be able to get ~9100 without too much hassle.

What others said is true. I love the red personally, but unless you have cream leather to contrast and the M Sport styling, red wouldn't be the colour I'd pick, especially if I knew I'd want to sell the car quickly. Black is the safe option people love these days.

I'm guessing yours is the 2.5l 325i too which is less desirable for tax reasons.